Numerous studies have evaluated both modern and ancient environments and have shown that size distributions of framboid diameters can be used to interpret the location of the redox boundary relative to the sediment-water interface at the time of framboid formation (e.g. Wilkin et al., 1996; Wilkin et al., 1997; Wignall and Newton, 1998; Bond and Wignall, 2010). In an environment with a euxinic/dysoxic-oxic boundary within the water column, framboid size distributions will have a narrow range with a small mean diameter. In environments where the same boundary is at or below the sediment-water interface, framboids are able to grow to larger sizes and typically have a wider range and a larger mean diameter (Wilkin et al., 1996).
Based on this premise, more than 12,000 framboids were measured at 90 sample depths and analyzed in order to interpret paleowater column chemistry through the deposition of the top of the Onondaga Formation and Union Springs member of the Marcellus Formation. Two cores were utilized – the Snow Shoe and the Kenny Erb – with the investigation focusing on the Snow Shoe, which is thought to represent the distal basin.
The two sample Kolmogorov–Smirnov test was used to compare framboid diameter statistics from all samples within each well, which facilitated discretization of groups of samples with similar characteristics. Based on the results of this exercise, nine groups were defined; five in the Snow Shoe well and four in the Kenny Erb well. It was found that these groups aligned well with major changes in geochemical attributes as well as observations of rock characteristics made both macroscopically and microscopically. Additionally, it was found that groups 2, 3, 4 and 5, (identified in the Snow Shoe well) were equivalent to groups 6, 7, 8, and 9 (identified in the Kenny Erb well), respectively.
Group 1 is from the Snow Shoe well and does not have an equivalent in the Kenny Erb well. This group is associated with the Onondaga Formation and was found to have a framboid size distribution similar to those described by Wilkin et al. (1997) for units deposited under an oxic water column in the Holocene Black Sea. Samples that belong to groups 2 and 6 are found primarily in the top of the Onondaga Formation. The framboid size distributions in these groups were comparable to those found in modern dysoxic-oxic environments discussed by Wilkin et al. (1996). Samples belonging to groups 3 and 7 are associated the transition between the Onondaga and Marcellus, and although they are mapped in the euxinic zone presented by Wilkin et al. (1996), framboid diameters are typically slightly larger than those reported by Wilkin et al. (1997) from Holocene Black Sea units deposited under a euxinic water column. Group 4 and 8 samples are typically found in the Lower Union Springs member and fall definitively into the euxinic zone presented by Wilkin et al. (1996), as do samples belonging to groups 5 and 9, which are typically found in the Upper Union Springs member. Interestingly, group 5 and 9 samples have smaller mean diameters and narrower size distributions than samples from groups 4 and 8.
Geochemical systems and observations outlined by Wendt et al. (2015) were used, in conjunction framboid size distribution observations, to establish the history of water column euxinia from the top of the Onondaga Formation through the Union Springs member of the Marcellus Formation. These metrics indicate that the top of the Onondaga Formation was deposited under a dysoxic to oxic water column, and that bottom water euxinia set in during the transition to the Union Springs member. During the deposition of the Lower Union Springs member, bottom waters were euxinic, but the redox boundary was dynamic and often approached the sediment-water interface. This period is also marked by high productivity and sediment starvation, which allowed for the creation of a highly condensed section. During the deposition of the Upper Union Springs member, framboid size distributions indicate that the redox boundary was established in the water column, away from the sediment-water interface, implying that sustained water column stratification persisted. Although these conditions differ from interpreted water column chemistry presented by Kohl et al. (2014), the results of this study support the overall sequence stratigraphic framework of Kohl et al. (2014).